This invention concerns glass-ceramic materials characterized by a nepheline crystal phase and by a low susceptibility to heating in a microwave field. It is particularly concerned with such materials possessing these and other characteristics to a sufficient extent that they are useful in dinnerware products.
A glass-ceramic (semicrystalline) material is a material, initially produced as a glass, in which a substantial portion of the glass has been crystallized in a crontrolled uniform manner. It is customarily produced by thermally treating a selected glass to grow crystals on nuclei formed from a nucleating agent included in the glass melt.
Glass-ceramic materials have found widespread product application ranging from missile nose cones to cooking surfaces. However, a major commercial impact has been in the housewares field, in particular, cookware and tableware.
My U.S. Pat. No. 3,201,266, granted Aug. 17, 1966, describes a narrow glass composition range in the Na.sub.2 O.BaO.Al.sub.2 O.sub.3.SiO.sub.2.TiO.sub.2 system. These glasses are capable of being thermally converted to glass-ceramics possessing a physical appearance and properties that render them eminently suitable for dinnerware.
The glass-ceramics are described as having a coefficient of thermal expansion in the range of 75.4-110.7.times.10.sup.-7 /.degree.C. and a crystal phase consisting essentially of nepheline and a dimorph of BaO.Al.sub.2 O.sub.3.2SiO.sub.2 selected from the group consisting of celsian and hexacelsian. They are also described as having crystals less than 30 microns in diameter, an average inherent abraded MOR that is greater than 12,000 psi and that may be increased to greater than 30,000 psi by glazing, and a capability of withstanding thermal shock from 140.degree. C. to 0.degree. C. Further, these glass-ceramics may have a color and feel equivalent to fine china, and may be highly resistant to food staining and detergent attack.
The rapid rise in microwave food preparation has imposed certain new requirements on cookware and dinnerware to enable use in such heating units. A particularly important requirement is microwave compatibility, that is, low absorption of microwave energy and consequent low susceptibility of a dish to become unduly hot during use. Most glasses and glass-ceramics are quite satisfactory for microwave use. However, those disclosed in my patent mentioned earlier are unusual in that they are highly susceptible to excessive heating in a microwave field. Hence, they are not well-suited for such use. Inasmuch as these materials have excellent properties in many other respects, it would be desirable to correct this deficiency without loss of such other properties.